This invention relates to a mold closing unit for an injection molding machine having a short working stroke and a longer servicing stroke, especially for production of disc-shaped parts.
In injection molding of flat parts such as compact discs, the opening Stoke for the production cycle of the injection molding machine should be kept only as large as necessary for unmolding. Such compact discs are a few millimeters thick. Thus, theoretically, an opening slightly larger than 1 cm would be sufficient. Recent trends have in fact been toward short strokes of 30 to 50 mm to minimize the so called dry run time. In changing molds, however the opening width must be increased relatively rapidly up to approx. 300 mm. This operation is known as a servicing stroke.
The data carriers known today as CDs have gained a key position in the economy and in also in the private sector that can no longer be overlooked. Digitized storage of data plus audio and video has set completely new standards. There are already known CDs which have a very high data density as well as new multilayer technologies with up to two information layers. MO (magnetic optical), CD-R (writable once, for photos, computer data, music/video recordings), CD-RW (multiply writable, for computer data and music recordings, video sequences), music CDs and CD-ROM. The constant increase in date volume and the associated data compression as well as the data storage technologies developed along with them demand ever greater precision, reproducibility and long-term stability iii the molding operations. This in turn makes maximum demands of the control rule accuracy of speeds, pressures, forces and temperatures for production of such data carrier discs by inject on molding. The constancy, especially in the technical process parameters listed above, deter nines the quality of the discs and has a significant influence on the profitability of the overall system of production installations. A modern production shop for mass production of CDs will include several production units, with a larger unit being able to achieve a production au put of up to 10,000 discs in 24 hours. Not only is the success of such production determined by the highest demands of the casting system as Such, but also it requires a precision interaction of the most important subsystems with respect to all the functions and quality features, primarily in involving the machine, the molds and handling as well as the removing device, delivery, etc. The highest possible reproducibility is demanded along with precision shaping of the pits and excellent long-term process stability with extremely short cycle times. A larger factory will frequently also have its own recording studios, a modern mastering infrastructure and several replication lines and will produce, for example, 60 million optical discs per year or 220,000 discs per day.
The enormous production figures listed here for just one factory necessitate a high degree of specialization. Although the market demands CDs with various basic structures, unlimited variation is required with respect to the specific data to be recorded on the CDs. Individual series are usually small, but they may amount to only a few thousand or tens of thousands. The result is unusually rapid changing of the mold inserts for injection molding machines. Under some circumstances, a mold or the corresponding stammer may be in the machine for only a quarter hour or a half hour, and then it must be replaced by another. In addition to error-free production, profitability involves mainly two factors which are almost equally important:
1. The cycle time for one CD including the required dry run time,
2. Setup time for insertion of the die (stamper) into the base mold for changing information.
The set-up time with series, which are usually small, has a great effect on productivity because stampers must often be changed several times an hour. The extreme requirements presented here are met very successfully today with fully hydraulic injection molding machines. Instead of the classical negative mold, so-called stampers are used for CD production. When seen from a distanco, stampers are a type of thin CD having the image of the surface structure of the CD as a negative mold. Thus, only the stamper plate is changed; it can be inserted into the mold and removed again in half a minute, for example. One advantage of electromechanical injection molding machines in comparison with hydraulic drives is that they have better utilization of the driving energy and sometimes even allow a higher precision for parts production. Another advantage of electric drives in addition to their low noise is that they do not use any oil hydraulics which are known to use high pressures, so this avoids any risk of soiling due to leakage of oil. Electromechanically operated injection molding machines must rely on force translating systems such as ball spindles, toothed racks, lever systems is, etc. for generating the closing force: such systems, such as a double-toggle system, increase the force on the crosshead or articulated head by a factor of 24 to 50, for example, in order to generate the closing force. However, today""s electromechanical injection molding machines having a toggle system are not suitable for short working strokes, because then lubrication of the hinge pins is no longer guaranteed. When working with fully hydraulic injection molding machines, however, a stroke distance of 70 mm is consisted with the state of the art.
The object of this invention is to develop a short stroke/long-stroke combination which will allow the greatest possible productivity and the highes: quality as well as the shortest possible dry run time for production of flat parts. Another goal is to develop a machine with the most compact possible design using one, two or more cavities, in particular also for use in a clean room.
This object is achieved according to this invention by the fact that it has a drive carrier plate having at least one electric motor servo axle, where the drive carrier plate can be connected to the machine stand of the injection molding machine in a stationary position at the end or it may move relative to the mold backing plate on the nozzle end.
The inventors have recognized that optimum use of electromechanical driving means, especially with regard to suitability for use in a clean room, can be guaranteed in particular if they are defined locally starting from a drive carrier plate instead of dividing the drive means between two carrier plates, as is the case currently with the most successful hydraulic CD machine.
This new solution permits some entirely now designs for both the short stroke or the working stroke and the long stroke or servicing stroke. A first embodiment is characterized in that it has an electric motor crank mechanism, preferably with a servo motor for the short stroke, by means of which a movable mold backing plate can be opened and closed quickly for the working stroke. The crank mechanism has an eccentric shaft and preferably two crank levers arranged in parallel, driven by a synchronous or asynchronous motor and acting symmetrically on the movable mold plate. The movable mold backing plate and the crank mechanism thus form a compact module anchored on the drive carrier plate, with the movable mold backing plate being guided on a guide. The enormous advantage achieved with a compact module consisting of the movable mold backing plate together with the crank mechanism is to be seen first of all in the production of an entire machine. The module permits economical modular assembly. If all the elements within one module are manufactured and fitted with the highest precision, this yields the highest precision for the overall functioning as well, despite the very short assembly time, because it can be based on the internal precision of the module. The crank mechanism can be utilized optimally in the crank function, as a pure impact function at the maximum required closing force by means of an eccentric shaft, and in the corresponding dead center range, the crank can be used for 100% unloading of the driving means or their bearings before this range. The greatest possible forces from the standpoint of mold closing can be reduced to purely static supporting forces which are more easily controlled per se. A not insignificant advantage is derived from the fact that not only any desired slot working strokes can be used, but also the short stroke is kept at least approximately in the dead center position when the crank mechanism is in the closed position. For the open position, however, it can be stopped in any desired position outside the dead center position to shorten and optimize the dry run time. To ensure the greatest possible closing force or the largest possible closed positions, the crank mechanism always moves at least approximately into a fixed preselected optimum position. Variations in mold thickness such as those due to temperature, for example, are adjusted by correcting the position of the mold backing plate on the nozzle end.
According to a second embodiment of this invention, the mold closing unit is characterized in that it has an electric motor column-type nut drive for the servicing stroke by means of which a mold backing plate on the nozzle end can be moved relative to the drive carrier plate for the servicing stroke. It is especially preferable here for the columns themselves to be drivable and for the mold backing plate on the nozzle end with the nuts mounted on it to be designed to be movable. Thus, the drive carrier plate becomes what is indicated by the phrase to the full sense of the term. The corresponding end plate becomes a carrier for the drives and is preferably fixedly connected to the machine stand. This brings several advantages at once. The static forces can be closed in the shortest distance. The highly dynamic forces are diverted or controlled from only one side, namely only from the drive carrier plate. In this way, possible vibrational processes can be kept under control more easily or compensated, while access can be ensured at the same time, even for corrections.
The rotational drive for all columns takes place centrally, preferably by means of a toothed belt drive or a rim gear. It is also proposed that the central column-type nut drive should be designed as a final controlling element of the closing force regulation together with a force sensor arranged on a machine part as an actual value generator for detection of the closing force and a control device. Position detection of the stroke movement takes place advantageously by means of position detection in the servo motors themselves. These reproducible positions are used by the control/regulation and coordination of the various axles, e.g., also for control/regulation and synchronization of the actuation of the removing device and the mold closing axle. The control/regulation device is designed as a central adjustment for the servicing stroke and the working stroke with the required memories for storing formulations and programs. The control/regulation device is equipped for precise, coordinated control/regulation of the servo axles for the short stroke as well as the long stroke and with corresponding servo boosters for synchronous or asynchronous motors. It is important here that it has at least one independently controllable drive motor, preferably a servo motor for the working stroke and the servicing stroke. In the normal case, both drive sides are designed electromechanically for the working stroke and for the servicing stroke, with the direction of impact of the crank mechanism being arranged at least approximately symmetrically inside the column axles. The movable mold backing plate and the mold backing plate on the nozzle end are guided on a common guide. The column-type nut drive has at least three driven columns, preferably four, with the overdrive for the three or four columns being arranged on the outer end of the drive carrier plate and the overdrive for the crank mechanism being arranged on the inner end of the drive carrier plate.
In a modification of the possible embodiments described above, the mold plates, e.g., also the short-stroke carrier plate and the long-stroke carrier plate, may each be designed as selectable fixed mold plates or movable mold plates for very specific applications, with a nut drive having a central axial drive again being provided here for the relative movement of the plates. All the movement functions are primarily movements of the components relative to one another and only secondarily are they movements of the components relative to the stationary machine stand. For example, if the long-stroke carrier plate is fixedly anchored on the machine stand, the movement functions of the injection end must be adapted and coordinated accordingly.